1. Field of the Invention
This invention relates to a sheet-fed double-sided document scanner and more particularly relates to a scanner scanning double sides of a document in different scanning modes according to different power levels of the power provided.
2. Description of the Prior Art
Usually, the double-sided document scanners could be classified into multi-pass scanners having an image-capturing assembly including an image sensor such as a CCD image sensor, and one-pass scanners having two image-capturing assemblies.
For the scan of a document by the one-pass scanner, the scanner transports a document past the two staggered image-capturing assemblies only once to complete the scan. However, because the multi-pass scanner has only one image-capturing assembly, the multi-pass scanner is required to be equipped with a sheet turning passage providing a sheet turning path for the document to return to the image-capturing assembly again. For the scan by the multi-pass scanner, the scanner scans the image of one side of the document when the document passes the image-capturing assembly at the first time. Then the document needs to be turned by the sheet turning passage to pass the image-capturing assembly again for completing the scan of the image of the other side of the document. In comparison, the scanning time of the one-pass scanner is shorter than that of the multi-pass scanner; on the contrary, the one-pass scanner activates the two image-capturing assemblies, so the power consumption is higher. In addition, although the power consumption for the multi-pass scanner is lower, the structure and sheet-transporting mechanism of the multi-pass scanner is more complicated.
As for the disposition of the scanner, the scanner is usually connected to a computer for transmitting a scanned image to the computer. A universal serial bus (USB) interface is adopted most often to be a connection interface. Furthermore, either the one-pass scanner or the multi-pass scanner has its own required driving power condition. If the working environment for the scanner is not able to provide the required driving power condition, the scanner will not work. Therefore, the conventional double-sided document scanner works only under a specific driving power condition. It lacks flexibility in use and is not user-friendly.